Our long-term goal is to characterize functionally and morphologically the chemosensors of the nervous system concerned in the homeostasis of brainstem functions. Specific objectives are: (1) to examine the role of postulated medullary pial receptors in the regulation of alveolar CO2 tension, (2) to identify the emetic chemosensor in the area postrema of the medulla oblongata, (3) to elucidate the chemosensory mechanism in the nodose ganglion of the vagus nerve. Each chemosensor will be probed in three ways designed to: (1) delineate its reflex motor characteristics, (2) obtain access to its discrete site of chemosensitivity, (3) control its sensed environment. Experiments will be performed acutely in decerebrated unanesthetized cats and chronically in cats prepared with sterile operative procedures. Techniques to be employed include surgical and chemical ablations, CSF perfusion, cross-circulation, radioautography and electrical activation and recording of neural discharges. Emphasis will be placed on effects of drugs on CO2 regulation dynamics, vomiting in opiate addition, and on modes of "afferent" drug actions.